1. Field of the Invention
The present invention relates to an anion concentration measuring device to measure anions, particularly chloride ions, in a biological component.
2. Description of the Related Art
Ion-selective electrode techniques are widely used as methods of analyzing the concentrations of biological substances in the medical field, particularly, electrolytes (sodium ions, potassium ions, and chloride ions, and the like) in serums and plasmas. In the ion-selective electrode techniques, the ion concentration in a sample can be measured by simply immersing an ion-selective electrode together with a reference electrode in the sample solution. Thus, the ion-selective electrode techniques are employed in automatic biochemical analyzers to make use of their advantages of achieving the miniaturization and automation of the analyzers.
Among the ion-selective electrodes, chloride ion-selective electrodes are classified into: a solid-state electrode made of a hardly-soluble chloride; and a liquid-membrane electrode to which an ion-exchange type ligand is added. A conventional liquid-membrane chloride-ion-selective electrode is formed in a way that a plastic film made of an epoxy resin and vinyl chloride is added with a plasticizer (e.g., dialkyladipate or dialkylphthalate) and a quaternary ammonium salt as a ligand. The quaternary ammonium salt used as the ligand is any of trioctylmethylammonium chloride, tridecylmethylammonium chloride and tetraoctylammonium chloride, for example. In addition, to enhance the improvement of the physical strength and the selectivity for chloride ions, there has been developed a chloride ion-selective electrode made of a copolymer of: a styrene monomer having a tri-long-chain alkylammonium group that is a ligand as a side chain; and a polymer having a hydrophilic group as a side chain (Japanese Patent Application Publication No. 2004-184365). On the other hand, to improve the physical strength and to prevent the elution of the ligand from deteriorating an electrode, there has also been developed an ion-selective electrode of a chemically modified glass film that uses, as a sensitive membrane, a porous glass to which a quaternary ammonium salt as a ligand is immobilized (Japanese Patent Application Publication No. Sho 61(1986)-170645).
An ion-selective electrode that includes a ligand immobilized on the surface of a gold electrode has been proposed in order to simplify the structure and the fabrication method of the above-mentioned ion-selective electrode and to achieve the miniaturization of analysis equipment in the future. Any of these ion-selective electrodes, however, is not capable of measuring an electric potential, and an ion concentration to be measured is obtained by impedance measurement. An example of a sodium ion-selective electrode has been reported in which a mercapto-crownether compound as a neutral-carrier type ligand is immobilized to a gold electrode (J. Am. Chem. Soc., 120 (1998) 4652-4657). In addition, examples of a lithium ion-selective electrode include an electrode in which a neutral-carrier type ligand is similarly immobilized to the surface of a metal electrode by utilization of bonding between thiol residue and gold (Anal. Chem., 78 (2006) 7132-7137, and WO2006/113440). Moreover, as anion selective electrodes, there has been an electrode in which a ligand for a fluoride ion (tetra-amide Calix [6] arene derivative) is immobilized on the surface of a gold electrode by utilization of bonding between thiol residue and gold (Langmuir, 2006, 22, 10732-10738).